► In view of quite recent experimental activities on magnetic and superconducting properties of honeycomb and hexagonal lattice based materials, in this thesis we have used…
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▼ In view of quite recent experimental activities on magnetic and superconducting properties of honeycomb and hexagonal lattice based materials, in this thesis we have used field-theoretic and many-body methods to investigate magnetic and superconducting properties of the large-U Hubbard model on the honeycomb lattice at half-filling and in the hole-doped regime. Within the framework of a functional-integral approach, we obtain the Lagrangian density associated with the charge (Grassmann fields) and spin [SU(2) gauge fields] degrees of freedom. The Hamiltonian related to the charge degrees of freedom is exactly diagonalized. In the strong-coupling regime, we derive a perturbative low-energy theory suitable to describe the (quantum) magnetic and superconducting phases at half-filling and in the hole-doped regime. At half-filling, we deal with the underlying spin degrees of freedom of the quantum antiferromagnetic (AF) Heisenberg model by employing a second-order spin-wave analysis, in which case we have calculated the ground-state energy and the staggered magnetization; the results are in very good agreement with previous studies. Further, in the continuum, we derive a nonlinear σ-model with a topological Hopf term that describes the AF-VBS (valence bond solid) competition. In the challenging hole-doped regime, our approach allows the derivation of a t-J Hamiltonian, and the analysis of the role played by charge and spin quantum fluctuations on the ground-state energy and, particularly, on the breakdown of the AF order at a critical hole doping; the results are benchmarked against recent Grassmann tensor product state simulations. In addition, we have performed an extensive study of the electronic structure of the doped system for each competing phase: AF, ferromagnetic (FM), and (spin-singlet pairing) s-, dx₂₋ʏ₂ – and idxʏ -wave superconducting (SC) state induced by purely electronic effects. In this context, an energetic analysis of the ground state of these phases reveal that the AF order prevails for low hole doping, while a dominantly chiral dx₂₋ʏ₂ + idxʏ superconducting state was found in the vicinity of the Van Hove singularity (high hole doping). We also stress that a thermodynamic analysis of the superconducting phase shows that the critical temperature is directly related to the exchange constant J = 4t²/U, in which t denotes the hopping amplitude and U the on-site Coulomb repulsion of the Hubbard model (purely electronic origin). Remarkably, the competition between the AF and dx₂₋ʏ₂ + idxʏ SC phases takes place by the occurrence of a first-order transition accompanied by a spatial phase separation of the referred phases.
Advisors/Committee Members: COUTINHO FILHO, Maurício Domingues (advisor), http://lattes.cnpq.br/4862700714316793 (advisor).

In this work we investigated entanglement in many-fermions systems. To explore inhomogeneous systems we proposed a local density approximation (LDA) for the single-site entanglement entropy. We analysed the universal and nonuniversal contributions to block-block entanglement and obtained an expression for the nonuniversal term. We employ a description in terms of the one-dimensional Hubbard model to investigate the entanglement in electronic nanostructures and to quantify the single-site entanglement with respect to the rest of the chain by means of the entanglement entropy. For the homogeneous Hubbard model we studied the entanglement behavior as a function of density, magnetization, electronic interaction and external magnetic fields. We found that the entanglement is sensitive to the metallic, insulating and superconducting phases. We observed an entanglement plateau in the region of the spin gap and verified that magnetic susceptibility and entanglement are intrinsically related. Energies and densities of the inhomogeneous Hubbard model, obtained from Density Functional Theory, combined with our proposal of an LDA for the entanglement entropy, were used to explore the behavior of the entanglement entropy in the presence of several inhomogeneities: superlattices, impurities and harmonic confinement. We verified that entanglement always decreases…

Electrons in narrow-band solids are strongly affected by the Coulomb interaction as well as lattice disorder. Both effects can lead to localization, but of different nature: correlation-induced Mott insulating state, and disorder-induced Anderson localization. The existence of the Mott phase is also significantly dependent on the band filling. Theoretical approaches to deal with this kind of system are usually based on the Hubbard Hamiltonian or related models, including disorder as a distribution of the on-site energies. In this work, utilizing Dynamic Mean Field Theory (DMFT), we study the Anderson- Falicov-Kimball and its three-band version, obtained as a simplification of the three-band Hubbard model associated to the CuO2 planes of high-critical-temperature cuprate superconductors, performing our analysis for the magnetic and non-magnetic cases. The one-particle density of states is…

Anomalous properties of the normal state of a strongly correlated electron system described by an attractive extended Hubbard model are investigated. The equations of motion of the Greens functions are calculated with the two-pole approximation which gives rise to quasiparticle renormalized bands. The two-pole approximation leads to a set of correlation functions. In particular, the antiferromagnetic correlation function h~Si ~Sji plays an important role as a source of anomalies in the normal state of the model. The uniform static magnetic susceptibility as a function of occupation nT and temperature is calculated. At low temperatures, the susceptibility presents a peak for nT ≃ 0.80. The results suggest that it is the onset of short range antiferromagnetic correlations, which could be a mechanism for the pseudogap. The Fermi surface, defined by the spectral function A(ω = 0,~k), is presented for different dopings. It has been observed that above nT ≃ 0.80 the ordinary Fermi surface evolves to a hole-pocket with pseudogaps near the antinodal points (0, π) and (π, 0).

Our main goal here is to analyze the low-energy properties of the two-dimensional (2d) Hubbard model slightly away from half-lling from a renormalization group point of view within the two-loop approximation. We will develop this technique adapting it conveniently to our problem at hand. As a warm-up test for our approach, we will apply the same methodology to the one-dimensional (1d) Hubbard model away from half-lling. As a result, we will show that this technique reproduces correctly the low-energy physics of the system, and the model is indeed described by the so-called Luttinger liquid. Next, we move on to the 2d problem. In this latter case, in a weak-coupling regime, we will observe that this model seems to be well described by Fermi liquid theory with quasiparticles adiabatically connected to the electrons in the noninteracting case. On the other hand, in an intermediate interacting regime, we will argue that our results are consistent with the interpretation of an insulating spin liquid (ISL) formation in the system with gaps in both charge and spin excitation spectra. This state would be an interesting example of a Non-Fermi…

[en] We investigated spin polarized transport
properties through a quantum dot connected with two terminals. An
electric current that circulates in our system can have its
polarization modulated with an external potential that controls the
spin orbit coupling (Rashba effect). We studied the effect of spin
polarization n a transistor constituted by a quantum dot where its
energies can be controlled with a gate potential that operates only
in the confinement region. The high confinement and correlation
between the charges give rises to interesting phenomena that we
describe in this work. We demonstrate that tuning an external
potential it is possible to control with a extremely efficient
precision the intensity and the polarization of the current through
this system. Another important parameter that we used to better
understand this system was the external electric field. In the
second part of this work, we studied the time evolution of a wave
function supposed to be initially a wave package circulating our
system composed by a quantum dot. We can prove resonant tunneling
effects and interference effects in such a wave package as time
goes by and we also studied spin orbit interaction effects on the
polarization of the carrier.

[en] In this dissertation we propose a projector
operator formalism to obtain the ground state energy of the
Impurity Anderson Hamiltonian with innite Coulomb repulsion U. This
formalism consists in projecting the Hilbert space into a sub-space
of one function corresponding to the ground state of the free Fermi
sea where a renormalized version of the Hamiltonian operates. The
ground state energy can be obtained through a self-consistent
process. From the knowledge of the energy, it is possible to
calculate the fundamental properties of the system as it is the
magnetization as a function of an external magnetic field, the
magnetic susceptibility, the dependence of the electronic
occupation as a function of the local energy of the impurity and
the Kondo temperature, which characterizes the universal behavior
of a Kondo problem.

Quantum bits, or qubits, are highly fragile due to interactions with the environment. The search for good protocols for protecting quantum information from decoherence is mandatory in order to make large-scale quantum computation possible. Most of the models proposed for this assume that correlations in the environment do not exist. Correlations can induce a time dependent error probability thus seriously damaging the quantum information over the time even if a quantum correction code is avaliable. In this way, we must taking into consideration possible physical limitations to fault-tolerant quantum computing. In this work we apply the Kernel Polynomial Method (KPM) to evaluate the density of states and fidelity decay of a L = 3 toric code without taking the lattice spin dynamics into account. The Hamiltonian model is based in a free bosonic environment and a spin-boson coupling, with two decoherence channels X and Z. A long-range, anisotropic interaction between spin pairs is then proposed as a correlated model. This correlation is directly related to the interaction strengh and range between spins. We show that the fidelity decay time scale depends on these parameters.

ntroduction: The use of drugs by pregnat adolescents is a serious problem of public health because it can harm the mother and the newborn. So,…
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ntroduction: The use of drugs by pregnat adolescents is a serious problem of public health because it can harm the mother and the newborn. So, the correct identification of the cases and the correlates of use of drugs in this population are important for the comprehension of this condition, for the identification of adolescents in high risk for this situation and for helping the development of public politics of prevention and correct treatment of these cases. Aims: 1) To check the validity of the self-report of drug use by pregnant adolescents, by comparing their responses to a structured interview about their use of cocaine and marijuana during the pregnancy with an analysis of their hair. 2) To investigate the association of cocaine and marijuana use during adolescent pregnancy in São Paulo, Brazil, with psychiatric disorders, social status and sexual history. Method: One thousand pregnant adolescents were evaluated using the Composite International Diagnostic Interview, and sociodemographic and socio-economic questionnaire at the obstetric center of a public hospital in São Paulo, Brazil. Hair sample was collected for analysis. Results: Hair analysis detected the use of cocaine and/or marijuana in the third trimester of pregnancy in 60 (6%) patients. 40 (4%) patients used only marijuana, 17 (1.7%) used only cocaine, and 3 (0.3%) used both drugs. None of the patients had reported the use of these substances in their interviews with healthcare professionals. The following are associated with cocaine and/or marijuana use during the third trimester of the pregnancy: Being younger than 14 years of age, having a history of more than 3 sexual partners, and having psychiatric disorders, specifically, bipolar disorder, post-traumatic stress disorder, and somatoform disorder. Conclusion: Although the prevalence of the use of drugs during pregnancy is significant despite consistent evidence about the compromise of the neurobehavioral development of the newborns that are exposed to drugs during the prenatal period, drug use is frequently not reported. Therefore, more sensitive methods of detection should be used so that appropriate medical and psychosocial interventions can be implemented for the mothers as well as for their children. In early adolescence pregnancy, having 3 or more sexual partners in life for this population is significantly associated with the use of cocaine or marijuana during gestation. This association suggests that specific intervention programs should target these young women.

Cross sections for fusion, strongly energy-damped processes and elastic scattering were measured for the 16 ,17, 18O + 10 11B and 19F + 9Be systems in the energy range 22 ELAB 64 HeV. Evidence that the fully energy-damped binary products observed in these reactions originate from a fusion-fission process, rather than through a deep-inelastic orbiting mechanism, is presented. The relative importance of the fusion-fission process in these very light systems is supported by experimental results which points towards the fission of a statistically equilibrated compound nucleus and also by model calculations. Charges and masses of the reaction products and evaporation residues were indentified using two ionization chamber and a time-of-flight system.

Only abstract. Paper copies of master’s theses are listed in the Helka database (http://www.helsinki.fi/helka). Electronic copies of master’s theses are either available as open access or only on thesis terminals in the Helsinki University Library.

► The dynamism and the speed at which companies transform themselves is increasing significantly, interfering in the tools used by managers to take decision. In this…
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▼ The dynamism and the speed at which companies transform themselves is increasing significantly, interfering in the tools used by managers to take decision. In this sense, the present research has as objective to identify the evidence of adherence of the artifacts of managerial accounting in relation to the stage of the life cycle of a digital company that showed an accelerated growth. The methodology used to obtain the results was the single case study, justified by the high complexity and important contribution to the understanding of the use of artifacts of management accounting in a given stage in the cycle of life. The survey was conducted in a company leader in the segment in which it operates by selling goods over the internet which showed a growth of 911% between 2007 to 2015. The model used for identification of the organizational life cycle was developed by Lester, Parnell and Carreher (2003) and for obtaining data were used documentary analysis, interviews and questionnaires, generating multiple sources of evidence were examined through a descriptive analysis of the data. The results showed that: (a) company studied is in a transition phase between the stage of existence and decline (decline); b) the artifacts of managerial accounting are classified as traditional tests used by the company are Strategic Planning, Budget, and analysis of budget : Profitability by Products and Panels of Indicators (Tableau de Bord). The intersection between the information indicated the use of artifacts that are consistent with those used by other organizations and a stage of life cycle of divergent current in relation to the artifacts of managerial accounting, timely proposals were submitted for the reduction of the gap between these two variables.
Advisors/Committee Members: PARISI, Claudio (advisor), http://lattes.cnpq.br/2891889803015460 (advisor).